The two actors play the Greek hero in separate movies, and the 'Hercules: The Legend Begins' star joked his body could never match up to The Rock's.
He told E! News: ''I wanted to get big, but then you have The Rock playing in the other Hercules. There's not enough 'extra-curricular' stuff for that.''
The 28-year-old star has still put himself through a tough and ''rigorous'' training regime to getting into top shape for the film.
He added: ''I wanted to put on weight. Renny [Harlin] the director, he just wanted me cut, so I had to get cut ... For me, it was very rigorous training.
''I had to learn how to ride a horse while wielding a sword and fighting mercenaries, as well as giving a badass speech and in the rain. And horses don't like the rain. It was very rigorous.''
Kellan also revealed while he can't match the 'Hercules: The Thracian Wars' star in terms of his physique, he's been glad to let himself indulge in some sweet treats on set.
He said: ''If you've ever been on a movie set, there's food all the time. It's hard to stay healthy because I had milk chocolate bars three times a day, and I'd just eat the whole bar.''
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